It is desirable to manufacture electrical interconnection systems from copper or a copper alloy due to the high electrical conductivity of copper and copper alloys. However, copper readily oxidizes to form compounds that are poor conductors, thus reducing its overall electrical conductivity. To prevent a gradual increase in its resistivity due to oxidation, a protective coating has been applied in the past, selected particularly from gels or metals such as gold and tin.
Copper is currently being considered as a potential metallization material for ultra-large scale integration applications because of its low electrical resistivity and good resistance to electromigration relative to the material currently used, i.e., aluminum or aluminum alloys. Unfortunately, as is well known, copper oxidizes rapidly to form an oxide which is neither a protective oxide nor is electrically conducting. In fact, the high reactivity of copper with its environment is one of the factors that limits the applicability of copper as an interconnect metal.
The problem of oxidation is especially severe in small copper or copper-containing particles, such as on the scale of nanoparticles, because the small particles have a large fraction of their atoms at or near the surface. Thus, the small particles are generally very sensitive to surface oxidation and contamination.